1. Field of the Invention
The present invention relates to driver circuits for light emitting diodes (LEDs).
2. Description of Related Art
LEDs are increasingly used in lighting applications, such as to provide back lighting for a liquid crystal display in which the LEDs are generally connected together in series in long strands. In such applications, it is desirable that the LEDs provide generally uniform illumination. Accordingly, it is necessary to closely regulate the current applied to the LED strands in order to maintain uniform illumination and provide efficient operation.
There are two methods generally used in the art to achieve constant current regulation of loads such as series-connected LEDs. The first method is to provide regulated output voltage and directly regulated load current. This method is often used in devices in which the control of the current must be very accurate and resistant to noise. A charge pump or boost converter generates a fixed supply voltage, and the LED strands are current-regulated from this voltage using respective linear current regulators. A drawback of this method is that the output voltage must be set conservatively high to account for device and temperature variation, resulting in wasted power and excess heat generated in the system. Where there are multiple strands with differing numbers of LEDs, the fixed output voltage must be set to the highest voltage requirement, thereby wasting a significant amount of power, particularly with respect to lower voltage strands having fewer series-connected LEDs.
The second method is to provide regulated current output and indirectly regulated output voltage. In this method, a voltage-feedback boost converter or charge pump provides an output voltage to the LED strands. The LED strands (i.e., load) are placed in series with a ballast resistor. The voltage across the ballast resistor is regulated by the boost converter or charge pump, thereby regulating the current through the load. This method has an advantage of seeking the minimum output voltage necessary to achieve the desired current. Its drawbacks stem from the fact that only one load current is directly regulated. Multiple strands each require separate ballast resistors to make up for the voltage mismatch in the LED strand loads. This results in less accurate control of current in the other LED strands. The LED strands also cannot be controlled independently or shut off when ballasted by resistors. Finally, in the case of differing numbers of LEDs in each strands, the strand with the highest voltage mismatch must drop the voltage mismatch across the ballast resistors, which wastes power.
In another type of implementation, multiple strands are current-regulated using linear current regulators, and an outer voltage regulator drives the output until the strand with the highest voltage drop load reaches a fixed reference voltage at the cathode of the LED strand. The disadvantage of this method is that the fixed reference voltage must be set conservatively high to account for the variation in the current regulator's voltage requirement due to process, temperature, and load variation.
By way of example, a conventional LED driver circuit may drive a first strand containing four white LEDs at 20 mA, for a total output voltage VOUT of 14V (i.e., 4×3.5V). The LED driver circuit may also support the occasional load of a second strand containing six white LEDs driven with 20 mA for a total output voltage VOUT of 21V. Since both loads may be driven at the same time, the minimum output voltage for a conventional resistor-ballasted, or fixed-output device must be greater than 21V to support either or both loads, taking into account ordinary lot-to-lot and temperature variation of the LEDs. Often, two inductive boost converters would have to be used, with each one driving a separate strand to its optimum efficiency point. This is not desirable, however, since the use of two converters is prohibitively expensive. As a result, most manufacturers resort to ballasting the first strand to above 21V using ballast resistors, causing constant loss of power dissipated in the ballast resistor that is not converted to light, reducing the overall efficiency of the system.
Accordingly, it is desirable to provide current regulation of LED strands in order to maintain uniform illumination and provide efficient operation, while overcoming the various drawbacks of the prior art.